An UWB radio is an RF communications device that uses high-bandwidth microwave pulses for transmitting digital data over a wide spectrum of frequency bands using very low power intensity. These devices can transmit data at very high rates (for wireless local area network applications) and very low rates (for telemetry applications). Within the power limit allowed under the current FCC regulations, UWB radios can carry large amounts of data over a short distance, at very low power. An UWB radio has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and at higher power levels. If higher power levels are used, UWB signals can travel significantly greater ranges.
UWB radio broadcasts use digital pulses timed very precisely. UWB radios do not use sine waves. The transmitter and receiver are highly synchronized, and they send and receive pulses with an accuracy of trillionths of a second. UWB technology is used in very high-resolution radars and precision (sub-centimeter) radio location systems.
Since UWB transmissions are high speed pulses instead of sine waves, UWB transmissions have a high degree of immunity to multipath fading. The constructive and destructive interference created by multiple reflections of the same sine wave signal being received simultaneously is known as multipath fading. This immunity to multipath fading makes UWB technology well suited for applications in environments that would otherwise suffer from multipath fading associated with sine wave transmissions.
UWB devices use the same increasingly crowded radio frequencies that many other systems use. UWB devices typically send out short electromagnetic pulses of half a billionth of a second, and the pulses are followed by pauses that of approximately 200 times that length. By spreading the pulses over a wide area of the spectrum (roughly 1 GHz), UWB devices use extremely low power and wide total bandwidths. UWB technology proponents envision applications such as home security and personal-area networks that activate home appliances. Police and fire departments are currently testing UWB devices that can detect people behind walls.
The government and private industry have an increasing interest in UWB technology. Two main UWB applications of interest include the transmission of large volumes of voice and data at very high speeds with relatively little power, and wall penetrating radars that provide detailed images of objects behind a wall. One of companies actively developing UWB applications is Time Domain Corporation. A representative example of Time Domain's technology is described in U.S. Pat. No. 6,031,862, by Fullerton et al, entitled “Ultrawide-Band Communication System and Method”. The voice and data applications, developed by Time Domain Corporation and others, have been primarily directed to increasing the transmission speed and volume of data, and the possibilities for using UWB transmissions for secure communications have not been fully exploited.
Accordingly, the present invention is directed to a method and apparatus for generating and detecting UWB or time domain impulsive communications signals which are useful in different types of secure communications systems.